quartz/content/notes/11-DNS.md

title	tags
11-DNS	lecture cosc301

[!INFO] hosts are named by ip addresses: e.g., 192.168.1.2 we want to map an ip address to a readable name so it is memorable in the past it was easy to store all the name of possible address in a folder /etc/hosts. it uses a NIC networking inforamtion system. but this is only for LAN impossible to store all IP addresses in the internet this way

[!INFO] DNS resolves a name to an IP address. can also do reverse mapping (this is useful for detecting spam emails).

[!INFO] distributed dns now dns is distributed. it is a BIND (berkely internet name domain) service. to do something distributedly. each LAN has their own NIS. each LAN manages their own domain names and IP addresses. LANs can talk between each other. we use a tree structure for domain names at the start we have a root domain ⇒ . then we have domains ⇒ .com, .nz, .org then sub domains ⇒ google.com, ocss.nz and sub sub ⇒ otago.ac.nz the advatage of tree structure is that the parent only need to know the domain name of the sub domains and the name server internal nodes are "zones" a "domain" is a leaf node.

[!INFO] google.com -> otago.ac.nz first find nameserver for .nz if they already know the address for .nz then can go directly otherwise they need to go through the root name server . you should configure this root Domain name server on a new installation the root name server send the address of the .nz name server which sends the address of the .ac.nz name server, and so on until you get the final address

[!INFO] nameserver storage some nameservers such as .nz are held by IANA

[!DEFINITION] NIS network information system

[!DEFINITION] DNS domain name service

[!INFO] DNS vs NIS dns is centralised, NIS is local

[!INFO] BIND windows -> WIND software to implement a DNS server "BIND (Berkeley Internet Name Domain) is a software collection of tools including the world's most widely used DNS (Domain Name System) server software. This feature-full implementation of DNS service and tools aims to be 100% standards-compliant and is; intended to serve as a reference architecture for DNS software."

[!INFO] Queries iterative query - doesn't give you the final answer only gives the name of the next server recursive query - only send the recurive query to final server. this must return the data requested for tell you that it doesn't exist. a LAN with an NIS will return recursive queries for all subdomain with that LAN

[!INFO] Files /etc/nsswitch /etc/hosts → contains local names, you can hardcode mappings in this file. this is more secure. if a domain name server if compromised you dont get the wrong name /etc/resolv.conf → should put address here? /etc/named.conf → root of the conf file. recruits other conf files. contains foward and reverse mappings /etc/named.root → contain the names and other information of root servers need to configure resolve order files->DNS

[!INFO] resource record when you contact nameserver you need to put resource record there? all names is record automaticall append parent domain name e.g., server1 becomes server1.otago.ac.nz resource records have different types this allow sthe BIND to know which is which need to set TTL of file. set the time for every record. Time to Live. e.g., 3 days. dns records are forgotten"flushed" after TTL has passed. after this they need to request the address again from the root nameserver if you change ip address you might have to wait for 3days for everyones record of you to be changed. while changing you should have two nameservers → one for the old and one for the new

[!INFO] types of server primary - one which most of the time is working secondary - backup we need high reliability, they can communicate between each other to update changes

Problem

• How to get the IP address with an IP name?
  • Mapping between IP addresses and IP names
• Simple solution
  • Central database, like /etc/hosts or Sun
    Microsystems’ NIS (Network Information
    Service) or Windows’ WINS for LAN.
  • E.g. tkm.govt.nz 13.77.63.24
• However, keeping billions of such records
  in a central database used by billions of
  users is almost impossible.

Domain Name Service (DNS)

• A distributed solution
• Each organisation, called domain, maintains
  its own database and answers queries about
  its domain.
• E.g.
  • oucs1120.otago.ac.nz 139.80.22.10
  • oucs1234.otago.ac.nz 139.80.34.67
  • chasm.otago.ac.nz 139.80.45.90
  • hextreme.otago.ac.nz 139.80.63.88